/*
 * Alloy Analyzer 4 -- Copyright (c) 2006-2008, Felix Chang
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */

package edu.ut.ece382c.valloy.ast;

import edu.mit.csail.sdg.alloy4.Err;


/**
 * This abstract class implements a Query visitor that walks over an Expr and its subnodes.
 * <br> As soon as one of the node returns a nonnull value,the nonnull value will be propagated to be the output of this visitor.
 *
 * <p> This default implementation will return null on all the leaf Expr nodes and thus the final answer will be null.
 * <br> To implement a particular query, you need to extend this class.
 */

public abstract class VisitQuery<T> extends VisitReturn<T> {

    /** Constructs a VisitQuery object. */
    public VisitQuery() { }

    /** Visits an ExprBinary node (A OP B) by calling accept() on A then B. */
    @Override public T visit(ExprBinary x) throws Err {
        T ans=x.left.accept(this);
        if (ans==null) ans=x.right.accept(this);
        return ans;
    }

    /** Visits an ExprList node F[X1,X2,X3..] by calling accept() on X1, X2, X3... */
    @Override public T visit(ExprList x) throws Err {
        for(Expr y:x.args) { T ans=y.accept(this); if (ans!=null) return ans; }
        return null;
    }

    /** Visits an ExprCall node F[X1,X2,X3..] by calling accept() on X1, X2, X3... */
    @Override public T visit(ExprCall x) throws Err {
        for(Expr y:x.args) { T ans=y.accept(this); if (ans!=null) return ans; }
        return null;
    }

    /** Visits an ExprConstant node (this default implementation simply returns null) */
    @Override public T visit(ExprConstant x) throws Err {
        return null;
    }

    /** Visits an ExprITE node (C => X else Y) by calling accept() on C, X, then Y. */
    @Override public T visit(ExprITE x) throws Err {
        T ans=x.cond.accept(this);
        if (ans==null) ans=x.left.accept(this);
        if (ans==null && x.right!=null) ans=x.right.accept(this);
        return ans;
    }

    /** Visits an ExprLet node (let a=x | y) by calling accept() on "a=x" then "y". */
    @Override public T visit(ExprLet x) throws Err {
        T ans = x.var.accept(this);
        if (ans==null) ans=x.sub.accept(this);
        return ans;
    }

    /** Visits an ExprQuant node (all a:X1, b:X2... | F) by calling accept() on a:X1, b:X2... and then on F. */
    @Override public T visit(ExprQuant x) throws Err {
        for(Expr y:x.vars) { T ans=y.accept(this); if (ans!=null) return ans; }
        return x.sub.accept(this);
    }

    /** Visits an ExprUnary node (OP X) by calling accept() on X. */
    @Override public T visit(ExprUnary x) throws Err {
        return x.sub.accept(this);
    }

    /** Visits an ExprVar node by calling accept on its associated subexpression. */
    @Override public T visit(ExprVar x) throws Err {
        return x.expr.accept(this);
    }

    /** Visits a Sig node (this default implementation simply returns null) */
    @Override public T visit(Sig x) throws Err {
        return null;
    }

}
